1. Field of the Invention
The present invention relates to a interdigital transducer for use in a transversal surface acoustic wave filter, a surface acoustic wave filter, and a radio communication apparatus.
2. Related Art of the Invention
As cellular phone terminals have become popular, their functions have been improved, while their sizes and power consumption have been reduced.
Under these circumstances, SAW (Surface Acoustic Wave) filters, characterized by consuming reduced power and having reduced sizes, have become essential for transmission and reception circuits of the cellular phone terminals.
FIG. 23 is a schematic view of a transversal SAW filter 407. The SAW filter 407 is mainly used for intermediate frequencies for a communication method such as CDMA.
The SAW filter 407 is composed of a piezoelectric substrate 401, an input IDT 402, an output IDT 403, an input terminal 404a, an input terminal 404b, an output terminal 405a, and an output terminal 405a. The IDT stands for an Interdigital Transducer.
Electric signals of intermediate frequencies input to the input terminals 404a, 404b as a pair of balanced terminals are converted into a surface acoustic wave by the input IDT 402. Then, the surface acoustic wave 406 propagates through the piezoelectric substrate 401 and is then converted into an electric signal again by the output IDT 403. The electric signal is output by the output terminals 405a, 405b as a pair of balanced terminals. In this manner, the SAW filter 407 operates as a transversal SAW filter.
FIG. 25 shows an IDT 1024 used as the input IDT 402.
The IDT 1024 is structured so that a plurality of electrode fingers are connected an upper bus bar electrode 224a and from a lower bus bar electrode 224b arranged opposite the upper bus bar electrode 224a. The upper bus bar electrode 224a and the lower bus bar electrode 224b are arranged parallel with a direction in which a surface acoustic wave propagates.
The IDT 1024 is composed of a section in which a surface acoustic wave is propagated in both directions and a section in which a surface acoustic wave is intensely propagated in one direction; the arrangement of the electrode fingers in the first section is different from that in the second section.
A bidirectional electrode 101 propagates a surface acoustic wave in two directions parallel with the upper bus bar electrode 224a and the lower bus bar electrode 224b. Further, a single phase unidirectional transducer (SPUDT) 102a located to the left of the bidirectional electrode 101 in the drawing intensely propagates a surface acoustic wave in a direction P in FIG. 25. Further, a single phase unidirectional transducer 102b located to the right of the bidirectional electrode 101 in the drawing intensely propagates a surface acoustic wave in the direction P in FIG. 25.
The bidirectional electrode 101 is structured so that a bidirectional electrode unit (a0)1 as a basic unit that propagates a surface acoustic wave in two directions parallel with the upper bus bar electrode 224a and a number of lower bus bar electrodes 224b are disposed at every distance equal to one wavelength of a surface acoustic wave at the predetermined frequency, and in FIG. 25, three successive bidirectional electrode units (a0) are disposed. The predetermined frequency of the surface acoustic wave is the center frequency of the surface acoustic wave excited on the piezoelectric substrate.
The bidirectional electrode units (a0)1 are each composed of four electrode fingers. The leftmost electrode finger in the drawing is connected the upper bus bar electrode 224a. The three other electrode fingers are connected the lower bus bar electrode 224b. 
These four electrodes each have a width equal to one eighth of the wavelength at the above mentioned predetermined frequency.
The single phase unidirectional transducers 102a and 102b are called “EWC-SPUDT electrodes” and utilize reflection of a surface acoustic wave therein to propagate this wave in one direction. These electrodes have hitherto been known as a method with a small loss. The single phase unidirectional transducer 102a is composed of a number of single phase unidirectional transducer units (a)2 as basic units which propagates a surface acoustic wave in the direction P and which are disposed at every distance equal to one wavelength that the predetermined frequency. Likewise, the single phase unidirectional transducer 102b is composed of a number of single phase unidirectional transducer units (a)2 as basic units which propagates a surface acoustic wave in the direction P and which are disposed every distance equal to one wavelength at the above mentioned predetermined frequency.
In FIG. 25, the single phase unidirectional transducer 102a is composed of two successive single phase unidirectional transducer units (a)2. The single phase unidirectional transducer 102b is also composed of two successive single phase unidirectional transducer units (a)2.
The single phase unidirectional transducer unit (a)2 is composed of three electrode fingers. The leftmost electrode finger in the drawing is connected the upper bus bar electrode 224a. The electrode finger located immediately to the right of the leftmost finger is connected the lower bus bar electrode 224a. The rightmost electrode finger in the drawing is connected the lower bus bar electrode 224b. Further, rightmost electrode finger is wider than the two other electrode fingers. For example, the rightmost electrode finger has a width equal to one-fourth of the wavelength at the predetermined frequency. The two other electrode fingers have a width equal to one-eighth of the wavelength at the predetermined frequency.
In this manner, the IDT 1024 is constructed so that the single phase unidirectional transducers 102a and 102b are arranged at the respective ends of the bidirectional electrode 101. For all basic units composed of the single phase unidirectional transducer units (a)2 and bidirectional electrode units (a0)1, the space between the excitation centers of adjacent basic units is a multiple of the above described wavelength. Accordingly, in the IDT 1024 as a whole, a surface acoustic wave is intensely propagated in the direction P in FIG. 25.
Accordingly, the IDT 1024 can be used as the input IDT 402 of the SAW filter 407 in FIG. 23 by connecting the input terminal 404a to the upper bus bar electrode 224a and connecting the input terminal 404b to the lower bus bar electrode 224b. 
Further, FIG. 26 shows an IDT 1025 used as the input IDT 402.
The IDT 1025 is structured so that single phase unidirectional transducers 112a and 112b are arranged at the respective sides of a bidirectional electrode 111. As with the IDT 1024 in FIG. 25, the bidirectional electrode 111 propagates a surface acoustic wave in two directions parallel with the upper bus bar electrode 224a and the lower bus bar electrode 224b. Further, both single phase unidirectional transducers 112a and 112b intensely propagate a surface acoustic wave in the direction P in FIG. 26 as with the IDT 1024 in FIG. 25.
The bidirectional electrode 111 is composed of three successive bidirectional electrode units (c0). The single phase unidirectional transducers 112a and 112b are each composed of two successive single phase unidirectional transducer units (c)12.
Bidirectional electrode units (c0)11 are each composed of four electrode fingers. The third electrode finger from the left in the drawing are connected the upper bus bar electrode 224a, and the other three electrode fingers are all connected the lower bus bar electrode 224b. Further, the single phase unidirectional transducer unit (c) 12 is composed of three electrode fingers. The leftmost electrode finger is connected the lower bus bar electrode 224b. The second electrode finger from the left is connected the upper bus bar electrode 224a. The third electrode finger from the left is connected the lower bus bar electrode 224b. Thus, the IDT 1025 has an arrangement of electrode fingers, i.e. basic units different from those of the IDT 1024. However, for all basic units composed of the single phase unidirectional transducer units (c)12 and bidirectional electrode units (c0)11, the space between the excitation centers of adjacent basic units is a multiple of the above described wavelength. Accordingly, in the IDT 1025 as a whole, a surface acoustic wave can be intensely propagated in the direction P in FIG. 26. The other arrangements are similar to those in FIG. 25.
Thus, the IDT 1025 can be used as the input IDT 402 of the SAW filter 407 in FIG. 23 by connecting the input terminal 404a to the upper bus bar electrode 224a and connecting the input terminal 404b to the lower bus bar electrode 224b. 
However, in the bidirectional electrode unit (a)1, a basic unit of the bidirectional electrode 101 of the IDT 1024 in FIG. 25, one electrode finger is connected the upper bus bar electrode 224a, while three electrode fingers are connected the lower bus bar electrode 224b. Similarly, in the bidirectional electrode unit (c0)11, a basic unit of the bidirectional electrode 111 of the IDT 1025 in FIG. 26, one electrode finger is connected the upper bus bar electrode 224a, while three electrode fingers are connected the lower bus bar electrode 224b. 
Thus, in the bidirectional electrode 101 of the IDT 1024, the number of electrode fingers connected the upper bus bar electrode 224a is different from the number of electrode fingers connected the lower bus bar electrode 224b. Only one electrode finger contributes to excitation derived from the upper bus bar electrode 224a. Thus, the electrode is inefficiently excited. Consequently, the use of the IDT 1024 results in a large loss to the SAW filter 407.
Similarly, in the bidirectional electrode 111 of the IDT 1025, the number of electrode fingers connected the upper bus bar electrode 224a is different from the number of electrode fingers connected the lower bus bar electrode 224b. Only one electrode finger contributes to excitation derived from the upper bus bar electrode 224a. Thus, the electrode is inefficiently excited. Consequently, the use of the IDT 1025 results in a large loss to the SAW filter 407.
That is, in IDTs comprising a combination of bidirectional electrodes and single phase unidirectional transducers, of the single phase unidirectional transducers the number of electrode fingers connected an upper bus bar electrode is different from the number of electrode fingers connected a lower bus bar electrode. Disadvantageously, only one electrode finger contributes to excitation derived from the upper bus bar electrode. Thus, the electrode is inefficiently excited.